With the amount of rainfall received in the past few weeks in the Midwest, it is not surprising that northern corn leaf blight and gray leaf spot diseases have begun to appear.

Pierce Paul, The Ohio State University Extension corn and small grains disease specialists, said northern corn leaf blight (NCLB) is showing up early in Ohio corn fields. He said there are three reasons the disease has developed this early in some corn fields. Three basic conditions must be satisfied: 1) the fungus (Exserohilum turcicum) must be present; 2) the hybrid planted must be very susceptible to the prevalent races of the fungus; 3) and environmental conditions must be highly favorable.seven

And Paul says this year may be the perfect storm:

Spore build-up + susceptible hybrid + extended periods of favorable weather conditions usually = major disease problems.

“So, unless, the weather becomes unfavorable over the next few weeks, this disease will continue to spread in fields planted with susceptible hybrids and will likely damage the upper leaves well before grain fill is complete. This would lead to substantial yield loss. When NCLB become established before silking, losses may be as high as 50 percent if favorable conditions persist. Continue to monitor the progress of the disease and the weather over the next 7 to 10 days and be prepared to treat susceptible fields with a fungicide in order to prevent the disease from spreading to the early leaf and leaves above the ear,” Paul wrote in a crop report published on June 30.

Carl Bradley, in his last report June 30 before leaving the University of Illinois as Extension plant pathologist, said both northern corn leaf blight and gray leaf spot were showing up in university research farms. He wrote about trials that the university has conducted using fungicides in previous years to control the diseases. He too emphasized that hybrids differ in their susceptibility to these diseases.

“If the rainy conditions continue, then a foliar fungicide application sometime between tassel emergence and silking may need to be considered on hybrids that are the most susceptible. Some general guidelines that may help make a foliar fungicide application decision follow:

  • “On susceptible to moderately-susceptible hybrids: consider a foliar fungicide if disease is present on the third leaf below the ear or higher on 50 percent of the plants prior to tasseling.
  • “On intermediate hybrids: consider a foliar fungicide if the field has a history of disease, if the previous crop was corn with at least 35 percent of the ground covered with residue, if disease is present on the third leaf below the ear or higher on 50 percent of the plants prior to tasseling, and if warm and humid weather has persisted.
  • On moderately-resistant to resistant hybrids: foliar fungicides generally are not recommended, but scouting is important to confirm that diseases are not present.

“The presence of diseases does make a difference in how profitable a fungicide application may or may not be. From trials conducted at the University of Illinois from 2008 to 2014 at many environments (45 total environments) in Illinois, the results indicate that the overall yield response to foliar fungicides was 5.3 bushels/acre. However, this yield response was 9.5 bushels per acre when disease developed to affect at least 10 percent of the leaf area in untreated controls (in 17 of the environments). In situations with low disease severity (disease developed to less than 10 percent of the leaf area in untreated controls), the average yield response was only 2.8 bushels per acre (in 28 of the environments). Obviously, the marketing price of corn and the fungicide and application costs will determine if fungicide applications were profitable. … The bottom line is that it takes a higher yield response to be profitable when corn marketing prices are lower.

Bradley is well known in the Midwest but left the University of Illinois for a similar Extension plant pathologist position with the University of Kentucky based at the Princeton Research and Education Center.